Exocytosis

Exocytosis is the process of moving materials from within a cell to the exterior of the cell. This process requires energy and is therefore a type of active transport. Exocytosis is an important process of plant and animal cells as it performs the opposite function of endocytosis. In endocytosis, substances that are external to a cell are brought into the cell.

In exocytosis, membrane-bound vesicles containing cellular molecules are transported to the cell membrane. The vesicles fuse with the cell membrane and expel their contents to the exterior of the cell. The process of exocytosis can be summarized in a few steps.
Vesicles containing molecules are transported from within the cell to the cell membrane.
The vesicle membrane attaches to the cell membrane.
Fusion of the vesicle membrane with the cell membrane releases the vesicle contents outside the cell.
There are three common pathways of exocytosis. One pathway, constitutive exocytosis, involves the regular secretion of molecules. This action is performed by all cells. Constitutive exocytosis functions to deliver membrane proteins and lipids to the cell's surface and to expel substances to the cell's exterior.

Regulated exocytosis relies on the presence of extracellular signals for the expulsion of materials within vesicles. Regulated exocytosis occurs commonly in secretory cells and not in all cell types. Secretory cells store products such as hormones, neurotransmitters, and digestive enzymes that are released only when triggered by extracellular signals. Secretory vesicles are not incorporated into the cell membrane but fuse only long enough to release their contents. Once the delivery has been made, the vesicles reform and return to the cytoplasm.

A third pathway for exocytosis in cells involves the fusion of vesicles with lysosomes. These organelles contain acid hydrolase enzymes that break down waste materials, microbes, and cellular debris. Lysosomes carry their digested material to the cell membrane where they fuse with the membrane and release their contents into the extracellular matrix.

2. ■exocytosis is a process that is used to transport
materials from inside the cell to the external
part of the cell by the use of energy. Therefore,
it is a type of active transport mechanism and it
is the opposite of endocytosis. Generally, in this
mechanism of exocytosis,a special vesicle
bound to the cell membrane, containing the
cellular particles will expel the cell content to
the external part of the cell.
Exocytosis

3. ■This mechanism has been used in the removal of
waste materials from the cell, transport
hormones, and proteins, for chemical signaling
between cells and the construction of the cell
membrane. The vesicles involved in exocytosis
are formed by the Golgi bodies, endosomes, and
the presynaptic neurons. These vesicles fuse
with the cell membrane and they can either be
complete or temporary vesicles, depending on
their function.

4. Vesicles

5. ■Normally after endocytic pinocytosis and
phagocytosis, the cell membrane gets
damaged, and therefore the exocytic
process functions to repair the cell
membrane by transporting proteins and
lipids to the membrane for repair
mechanisms.

6. ■It is also the end-point mechanism
for the transportation of protein
complexes and packaging them in
their destined sites and insertion to
the cell membrane.

7. Exocytosis mechanism in Summary
■The transportation of the cell vesicle
containing molecules from inside the cell to
the cell membrane.
■The vesicle then attaches to the cell
membrane.
■During vesicle fusion with the cell
membrane, it allows the release of the
contents in the vesicle to the outside of the
cell.

9. Exocytotic Vesicle
■The exocytotic vesicle contains protein elements
that are acquired from the Golgi complexes,
which are the recipients of the proteins and
lipids that are synthesized in the Endoplasmic
reticulum.
■The Golgi complexes’ function is to sort and
modify proteins and stored within the secretory
vesicle, which buds off at the trans face of the
Golgi apparatus.

10. ■However, not all vesicles that fuse with the cell
membrane gain contact with the Golgi apparatus,
some are formed from the early endosome, a
membrane sac found in the cell cytoplasm. The
vesicle fuses with the early endosome by endocytosis
of the cell membrane, sorting the internalized
elements into proteins, lipids, microbe, and directs
them to their destinations, using transport vesicles.
The transport vesicles then bud off from the early
endosome, transporting the proteins and lipids to the
cell membrane and waste materials to the lysosomes
where they are degraded.

11. ■Another type os vesicles that do not originate
from the Golgi complexes are those found on
the synaptic terminals of the neurons.

12. Types of Exocytosis
■There are three pathways involved in
the exocytotic process.
■Constitutive exocytosis
■Regulated exocytosis
■Lysosome mediated exocytosis

13. Constructive exocytosis
■This type of exocytosis involves the delivery
of membrane proteins and lipids to the cell
membrane and to also get rid of substances
from the cell into the exterior. This is the
most common pathway that is performed
by all body cells.

14. Regulated Exocytosis
■This mechanism is common among the secretory cells
whose function is to store hormones, digestive
enzymes, and neurotransmitters.
■The secretion of these products must be triggered by
extracellular signals to initiate the formation of
secretory vesicles, which fuse with the cell membrane
for a long time to allow the release of the cell contents
out of the cell, into the exterior. After delivery, the
vesicles are reformed and returned to the cytoplasm.

16. Lysosome mediated Exocytosis
■This process involves the fusion of cell
vesicles with the cell lysosomes. Lysosomes
contain digestive enzymes and hydrolase
enzymes whose function involves the
breakdown of cellular waste materials,
microorganisms, and debris. The lysosome
carries the elements that have been broken
down onto the cell membrane where it
fuses with the cell membrane releasing its
elements into the extracellular cell matrix.

18. Exocytosis Examples
■A good example of exocytosis in the
transportation of glucagon from the
pancreas in theIslets of Langerhansinto
the liver where they are broken down into
glycogen which is further broken down to
glucose which is simple for absorption. The
glucose is then released into the
bloodstream.

19. ■In the islets of Langerhans, the glucagon
and insulin are stored in the secretory
vesicles of the pancreas. When the glucose
levels in the bloodstream are low, glucagon
is secreted by the islets alpha cells, carried
by the secretory vesicles into the liver cells
where they are released by exocytosis for
further processing and utilization.
■The pancreas also releases some digestive
enzymes by exocytosis.

21. ■Other exocytotic processes include the
synaptic vesicle exocytosis where a synaptic
vesicle filled with neurotransmitters in the
pre-synaptic neuron fuses with the pre-
synaptic membrane releasing
neurotransmitters into the synaptic cleft
(the gap between neurons). The
neurotransmitters can then bind to
receptors on the post-synaptic neuron.

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